1. Field of the Invention
The present invention relates generally to optical communication systems, and more particularly to a system and method for providing an eye safe laser communication system.
2. Related Art
Recently, there has been tremendous growth in, the deployment of fiber-optic facilities by telecommunication carriers such as Regional Bell Operating Companies (RBOCs), cable carriers, and Competitive Local Exchange Carriers (CLECs). Deployment of these fiber-optic facilities, along with the introduction of technologies such as OC-192 and DWDM, has dramatically lowered the marginal cost of bandwidth on the fiber.
As a result of this development, there is extensive bandwidth and communications capability in a telecommunication backbone networks. Many homes, offices and other facilities, however, do not have a practical solution to interface with these high bandwidth backbone networks. Consequently, direct attachment of potential customers to these backbone networks remains very expensive.
For example, two traditional methods have been employed for directly attaching to these backbone networks. These methods include buried or aerial fiber interconnections, and wireless microwave connections. Both of these methods incur significant up-front costs before any revenue can be realized. For example, in the case of buried or aerial fiber, significant costs are associated with obtaining rights-of-way for the cable runs, and installing the cable by burying or hanging. In the case of a microwave system, significant up-front costs are associated with the microwave repeater equipment, and obtaining rights to suitable portions of the spectrum. Therefore, system developers and integrators have sought long and hard to find suitable solutions to this so called xe2x80x9clast milexe2x80x9d problem.
Solutions to this problem are fully described in the related patent applications listed above. These solutions relate to a wireless optical communication network. The wireless communication network is typically configured to couple a backbone communication network to a plurality of users at a plurality of facilities. The wireless communication network typically includes a plurality of wireless network nodes. Each network node includes a wireless receiver and transmitter configured to provide an optical communication link with one or more network nodes. The transmitter/receiver pairs are disposed on a movable mounting structure that includes a motor drive mechanism to move the mounting structure in at least two dimensions for pointing the wireless optical transceiver to the designated network node(s).
A communication interface is typically included to provide communications between the wireless communication network and one or more facilities in close proximity to the node. At least one of the nodes can also include a second communications interface configured to couple the backbone communication network to the wireless communication network.
Typically, the network nodes are installed on top of buildings, towers or the like. In a typical implementation, the optical transceivers comprise a receiver that includes either a PIN diode or an avalanche photodetector diode. The optical transmitter typically includes an infrared laser diode with an output wavelength centered at approximately 780 nm.
Generally, the laser output generated by these systems is classified as xe2x80x9ceye safe,xe2x80x9d which translates into a class 1 rating in accordance with ANSI Z.136 standards. Accordingly, no eye damage would result by looking directly into the aperture of one of these network node devices. In one embodiment, the class 1 rating is partly the result of a translucent shroud used to house each network node. Typically, the translucent shroud is comprised of an acrylic cylindrically shaped housing that surrounds the network node. In this embodiment, the distance between the laser and the housing surface is far enough to decrease the density of the laser to eye safe levels. Thus, even if someone were to put their eye right up against the shroud, no eye damage would occur.
The problem is, however, eye damage could result if an optical focusing device, such as a pair of binoculars, a telescope or the like, were used to look into the aperture of a network node. That is, even though the laser is eye safe to an unaided eye, it is not necessarily eye safe to an aided eye. Therefore, what is needed is a system and method for providing a wireless laser communication system that is eye safe even if an external optical focusing device is introduced into the optical path.
Accordingly, the system and method of the present invention provides a means for producing a laser communication system that is eye safe even when external devices are introduced into the optical path. In a preferred embodiment this is accomplished by detecting an interfering object in the optical path and cutting off the communication beam or reducing the power of the communication beam to a safe level.
In one embodiment, the present invention provides a second laser diode transmitter and receiver within each network node. The second transmitter and receiver are separate and distinct from the transmitter and receiver used for the communication channel""s beam.
The additional laser diode is preferably a pulsed laser diode with a relatively high peak power, but a very low average power. A long duty cycle associated with the pulsed laser diode prevents the laser output from exceeding eye safety limits. The additional receiver can be any suitable receiver. In one embodiment the additional receiver consists of a monitor photodiode, which may be contained in the same package as the additional laser diode. However, it should be noted that any receiver that meets the necessary spectral sensitivity requirements could be used.
The laser is pointed at a corresponding node and the transmitted pulses travel in parallel with the communication beam. The transmitted pulses are reflected from the opposite network node and return to transmitting node within a known fixed time period. This known fixed time period of the reflected beam is referred to herein as the time of flight. The receiver in the transmitting node receives the returned pulses.
Because the beam from the pulse laser is parallel and in close proximity to the communication beam, objects that interfere with the communication link also interfere with the pulse laser beam. When an interfering object is present, in a first scenario, the pulses bounce off the interfering object and return to the receiver in the transmitting node. When a time of flight is less than the known fixed time of flight, an interference is detected.
In a seconds scenario, when an interfering object is present, the object absorbs the energy from the pulse laser and a reflected pulse is not detected. Accordingly, to account for both the above scenarios, the present invention determines whether the reflected pulse is detected within a window of expectation. If the reflected pulse is not detected within the window of expectation, an interference is detected. In response to this detection, a signal is sent to the transmitter to shut down the communication beam. In one embodiment, the flight time is determined with the use of a timer that is initiated concurrently with each a pulse transmission.
Other methods of detecting an interference can also be implemented. For example, in another embodiment, a CCD camera is used to capture a reflection from an object in the path of the communication link. In this embodiment the CCD has an overlapping field of view with the communication transceiver""s transmit laser. The reflection of the laser""s transmission forms a baseline pattern on the CCD. Any pixel to pixel variation from the baseline pattern indicates an interference, thus triggering the system to shut down or reduce the power of the communication link.
It is noted that the pixel to pixel variation caused by an object interfering with the communications link is discernable from a change in weather conditions. This is important because the former requires a reduction in power or shut down, while the latter requires an increased power level. These conditions are discernable by noting that a change in the weather, for example fog, would cause a uniform change in the CCD pattern. Conversely, an interference would cause a non-uniform change in the CCD pattern. Therefore, weather phenomenon can be distinguished from object interference.
The CCD preferably has an infinite depth of field, but this consideration requires a trade off depending on the characteristics of the overall system and a given link. For example, one implementation uses pinhole optics to create an infinite depth of field. However, less light is collected in this implementation. Therefore, the amount of return energy in the reflected signal is a key parameter in deciding whether this type of implementation is acceptable. In another implementation, a long focal length lens is used to increase the depth of field.
Another embodiment uses retroreflection of the human eye to detect interference by the human eye and an optical magnification device. In this embodiment, a CCD is used to detect signals bent back by retroreflection of the human eye. When such a signal is detected, the link is immediately shut down.
Another embodiment of the present invention utilizes the communication link itself. If an instantaneous power drop is detected, the system determines that an interference has occurred and shuts down or reduces the power of the communication beam.
Further features and advantages of this invention as well as the structure and operation of various embodiments are described in detail below with reference to the accompanying drawings.